The Author(s) 2018. This article is published with open access at www.chitkara.edu.in/publications.

ABSTRACT

Metal halide perovskites have shown to be a structure with great promise as an efficient photovoltaic, but at the same time it is affected by instability problems that degrade their performance. Degradation mechanisms vary with temperature, moisture, oxidation, and energy conversion during light exposure. We study performance loss due to temperature by probing diffusion of elemental composition across the thickness of films produced by spin coating and for temperatures ranging from 20 to 200°C. X-ray reflectivity was used to identify the electron density, composition, and quality of the films, aided with X-ray fluorescence and X-ray photoelectron spectroscopy studies to obtain information about degradation of the organic phase of the films.

INTRODUCTION

Perovskite solar cells (PSC) saw their introduction to the modern world of photovoltaics as a result of research in dye sensitized solar cells [1]. The efficiency of earlier PSC was about 3% [2] and most recently have increased to about 22% [3]. Efficiencies achieved in the lab are comparable to those of commonly known Si photocells [4]. Together with their efficiency, the main advantage of PSC is the low manufacturing cost. PSC are readily manufactured with spin coating processes [5, 6]. However, such processes present scalability difficulties when compared to those employed in modern semiconductor facilities. Possibly the main hurdle with PSC are their susceptibility to degradation. The basic structure of PSC, as is well known, corresponds to the formula ABX3, where, in the case here presented, A = methylammonium (MAI), B = Pb, and X = I, i.e., with formulation CH3NH3PbI3. Other combinations of materials are being researched by other groups to tune the bandgap [7] or to try to mitigate the toxicity of the compounds [8, 9]. We are addressing the effect of temperature-induced degradation only for CH3NH3PbI3 deposited on soda-lime glass substrates. The design of the solar cells studied here was based on initial estimations [5, 6], which suggested thicknesses ranging from 150 to 500 nm. However, experimentally the thickness started at 16 nm. The thickness is comparable to the exciton diffusion length, which is advantageous, without ignoring that photon absorbance due to thickness is an important factor. However we seek to explore efficiency of the structure on a diffusion-length-like thickness realm.